In recent years, standards on passenger car collision and pedestrian protection have been tightened, while improvement of the fuel efficiency and kinematic performances has been demanded. For example, a higher strength of bumper reinforces (bumper reinforcement), a higher energy absorbing efficiency and a lighter weight of bumper stays (crash box), and a higher energy absorbing efficiency of pedestrian protective components (see JP 2011-105183 A) are in increasing demand. For trucks, to prevent a passenger car from underrunning, collision strength standards for front and rear underrun protectors (see JP 2008-273271 A) are tightened, and on the other hand, due to an increase in laden weight, higher energy absorbing efficiency and a lighter weight of the underrun protectors and underrun protector stays have been demanded.
For the improvement of the energy absorbing efficiency, weight reduction, the reduction of the number of components, and simplification of a bonding method, there is provided a longitudinal-collapse-type bumper stay having a flange formed by applying electromagnetic molding at one or both ends of a light-weighted tubular aluminum alloy extruded material (or an aluminum alloy plate molded into a tube) (see JP 2010-116129 A, JP 2010-69927 A, JP 2006-305587 A, JP 2005-152920 A, JP 2005-7475 A, JP 2004-189062 A).
The diameter and thickness of the aluminum alloy extruded material is determined depending on a predetermined energy absorbing amount that should be absorbed using the bumper stay at collision. However, since the flange is expanded by tube expansion through electromagnetic molding, when the tube expansion rate (circumferential length after tube expansion/original circumferential length) is large, a crack may occur on the outer circumference of the flange, or the thickness may decrease, there is a limit in the width of the flange formed by electromagnetic molding.
Specifically, applied to mold various energy absorbing members such as bumper stays is an electromagnetic molding method of disposing a metal tubular material made of an aluminum alloy in a die having the inner circumferential face with a circular or polygonal cross section, inserting an electromagnetic molding coil into the metal tubular material, passing a large current through the electromagnetic molding coil in this state, inducing an induction current in the metal tubular material by electromagnetic inductive phenomenon, thereby causing an electromagnetic force according to Fleming's left-hand rule to expand the metal tubular material into a cross-sectional shape along the inner circumferential face and/or end face of the die.
For example, JP 2004-189062 A discloses that an end of an aluminum alloy tubular material is expanded to mold a bumper stay with a flange. JP 2004-237818 A describes that a front portion of an aluminum alloy tubular material is inserted into a through hole in a bumper reinforce, the periphery of a rear portion is surrounded with a die, and the whole length of the aluminum alloy tubular material is expanded to be swaged to the bumper reinforce. JP 2010-69927 A discloses that a rear portion of an aluminum alloy tubular material is expanded in first tube expansion while forming a flange in the rear end thereof, and a front portion is inserted into a through hole in a bumper reinforce in second tube expansion to swage the bumper reinforce. JP H6-31226 A discloses that an aluminum alloy tubular material having a circular cross section is expansion-molded into the material having an oddly-shaped cross section including polygonal cross section.